1. Field of the Invention
This invention relates to an image display system based on three-dimensional image data, and more particularly to an image display system which displays a tomogram along an arbitrary cross section.
2. Description of the Related Art
In order to observe the inside of an object, conventionally a plurality of tomograms are taken at predetermined intervals along an axis, for instance, by CT and are displayed. Recently, in order to display the inside of an object to be more understandable, there have been proposed a technology in which a plurality of pieces of image data (obtained, for instance, by CT) representing a plurality of cross sections are superimposed to obtain image data of a three-dimensional region of the object, and a tomogram of the object sectioned along a cross-section passing through a predetermined axis is displayed on the basis of the image data of the three-dimensional region of the object (see, for instance, U.S. Laid-Open No. 20020015006, or a technology in which a tomogram of the object sectioned along an arbitrary cross section (see, for instance, Japanese Unexamined Patent Publication No. 10(1998)-137236). Further, there has been proposed a technology in which a plurality of pieces of image data are subjected to weighted average processing and a weighted-average tomogram, in other words, “a tomogram provided with a depth” is displayed. (See, for instance, Japanese Unexamined Patent Publication No. 9(1997)-238934.)
According to these technologies, the user can easily grasp a three-dimensional image of the inside of an object since a tomogram along an arbitrary cross section of the object can be displayed and at the same time, the tomogram includes, as well as information on the designated cross section, information on cross sections before and behind the designated cross section.
In accordance with such a technology of displaying a tomogram having depth, the density information of the image to be displayed generally varies depending on the region to be displayed as an image. Accordingly, when the cross section and/or the depth of the tomogram to be displayed are changed, there is sometimes displayed a tomogram which is low in density and/or contrast and is hard to observe. Such a tomogram generally requires image processing in order to make it observable. Conventionally, the image processing has been manually carried out by the user. That is, each time the cross section and/or the depth of the tomogram to be displayed are changed and a tomogram hard to observe is displayed, it is necessary for the user to set image processing conditions and carry out image processing on the tomogram under the image processing conditions. However, these operations are troublesome for the user and make it infeasible to efficiently observe tomograms.